LEGUTIANO, Spain -- Spanish officials said a civil guardsman was killed and four wounded Wednesday when a car bomb exploded at a barracks in the Basque region of northern Spain.

Although there was no immediate claim of responsibility, authorities blamed the Basque separatist group ETA, said CNN, which reported the blast happened without warning at around 3 a.m. in Legutiano, Spain.

"We want to make it clear to the ETA terrorist group that all of the democratic parties are closing ranks to finish off the ETA terrorist group," Socialist Party majority leader Jose Antonio Alonso told a Spanish radio network. "I express my condolences to the families of the victims."

Authorities said Wednesday's killing was the sixth that has come as a result of ETA violence since the separatist movement ended a unilateral cease-fire in December 2006 with a Madrid airport bombing, CNN said.